Vehicle rollover detection

ABSTRACT

A system and method for detecting a rollover of a vehicle that includes at least one wheel reaction force sensing device for transmitting wheel reaction force signal indicative of an amount of force exerted on at least one wheel of the vehicle is provided. The system includes a controller operably coupled to the at least one wheel reaction force sensing device and including at least one accelerometer sensor for transmitting the acceleration signal. The controller is configured to determine a first force index in response to the wheel reaction force signal, determine a first lateral acceleration of the vehicle in response to the acceleration signal, compare the first force index to a threshold force index and the first lateral acceleration to a threshold lateral acceleration, and deploy a restraint system based on the comparison.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/125,091 filed May 22, 2008, the disclosure of which is incorporatedin its entirety by reference herein.

TECHNICAL FIELD

The embodiments of the present invention generally relate to a vehiclerollover accident detection.

BACKGROUND

The number of fatal accidents each year in the U.S. has hovered at about40,000 for a decade. Safety organizations, the government, and industryare working diligently to reduce that number to 30,000. Rollover basedaccidents account for about 30% of light-vehicle fatal accidents. Aportion of vehicle rollovers may be attributed to ‘hard trips’ where avehicle enters into a rollover state after traveling over a curb orobstacle. Other such vehicle rollovers may be attributed to ‘soft trips’where a vehicle enters into a rollover state after traveling over sandor grass lands.

In view of the number of fatalities associated with rollover accidents,original equipment manufacturers (OEMs) are continuing to developsensing algorithms to detect vehicle rollovers and implementing variousadvanced restraint systems to mitigate injuries of occupants from beingejected while the vehicle encounters a roll over event.

SUMMARY

In at least one embodiment, a system for detecting a rollover of avehicle that includes at least one wheel reaction force sensing devicepositioned about at least one wheel of the vehicle for transmitting awheel reaction force signal indicative of an amount of force exerted onthe at least one of the wheels of the vehicle is provided. The systemincludes a controller operably coupled to the at least one wheelreaction force sensing device and at least one accelerometer sensor fortransmitting an acceleration signal indicative of vehicle bodyacceleration about at least one axis of the vehicle. The controller isconfigured to determine a first force index in response to the wheelreaction force signal, determine a first lateral acceleration of thevehicle in response to the acceleration signal, compare the first forceindex to a threshold force index and the first lateral acceleration to athreshold lateral acceleration, and deploy a restraint system based on acomparison of the first force index to the threshold force index and thelateral acceleration to the threshold lateral acceleration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a quarter-model vehicle suspension in a steady-statecondition;

FIG. 2 depicts a roll reaction on a suspended vehicle;

FIG. 3 depicts wheel reaction forces in accordance to one embodiment ofthe present invention;

FIG. 4 depicts a rollover detection system in accordance to oneembodiment of the present invention;

FIG. 5 depicts a first rollover detection scheme in accordance to oneembodiment of the present invention;

FIG. 6 depicts a force index and lateral acceleration threshold plot inaccordance to one embodiment of the present invention;

FIG. 7 depicts a second rollover detection scheme in accordance to oneembodiment of the present invention;

FIG. 8 depicts a force index and roll rate threshold plot in accordanceto one embodiment of the present invention; and

FIG. 9 depicts a force index and roll angle plot in accordance to oneembodiment of the present invention.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

Referring now to FIG. 1, a quarter-model vehicle suspension 10 in asteady-state condition is shown. The dynamic behavior for thequarter-model vehicle suspension 10 may be obtained by applying Newton'ssecond law for the sprung and unsprung mass as shown in FIG. 1. Forexample, the equation of motion for the unsprung mass is:

M{umlaut over (Z)} _(u) +C _(s) Ż _(u)+(K _(s) K _(t))Z _(u) =C _(s) Ż+K_(s) Z+K _(t) Z _(r) +F _(W)  (EQ. 1)

where,

-   -   M=mass,    -   Z=sprung mass displacement,    -   Z_(u)=unsprung mass displacement,    -   Z_(r)=road displacement,    -   F_(W)=force on the unsprung mass,    -   K_(s)=suspension stiffness,    -   K_(t)=wheel stiffness, and    -   C_(s)=suspension damping coefficient.        EQ. 1 may be simplified by neglecting higher order items. In        light of such, the force on the unsprung mass may be rewritten        as:

F _(W) ≈f(Z,Z _(u) ,Z _(r))=(K _(s) +K _(t))Z _(u) −K _(s) Z−K _(t) Z_(r) +errs  (EQ. 2)

In general, the loads (or force on the unsprung mass) are equivalentbetween left and right sides of the vehicle when the vehicle is on ahorizontal surface with steady movement. However, the loads on theunsprung mass are different (e.g., between the left and right side ofthe vehicle) when the vehicle experiences a rollover event. In such acase, the vehicle may be unstable and lean to roll one side of thevehicle. The load on the leaned side of the vehicle may be high, whilethe load on the other side of the vehicle may be close to zero.

Referring now to FIG. 2, a roll reaction on a suspended vehicle 12 isgenerally shown. The suspended vehicle 12 includes a body section 14that is represented by a mass, m. A plurality of springs 16 and 18 arecoupled to the body section 14. The springs 16, 18 couple the bodysection 14 to an axle 20 having wheels 22, 24. The body section 14 ofthe vehicle 12 includes a roll center which provides a pivot point forthe body section 14 in which lateral forces are transferred from theaxle 20 to the mass. By taking moments about a point where the wheel 24contacts the ground and assuming the trailing side load of the wheel 22(e.g., wheel off of the ground) is equal to zero provides the following:

$\begin{matrix}{{\sum M_{o}} = {0 = {{m\; a_{y}h} - {m\; {g\left\lbrack {\frac{t}{2} - {\varphi \left( {h - h_{r}} \right)}} \right\rbrack}}}}} & \left( {{EQ}.\mspace{14mu} 3} \right)\end{matrix}$

From EQ. 3, the lateral acceleration

$\left( {{e.g.},\frac{a_{y}}{g}} \right)$

is found to be:

$\begin{matrix}{\frac{a_{y}}{g} = {\frac{t}{2h} - {\varphi \left( {1 - \frac{h_{r}}{h}} \right)}}} & \left( {{EQ}.\mspace{14mu} 4} \right)\end{matrix}$

As shown in EQ. 4, an unstable lateral acceleration generally depends onvehicle track width t, center of gravity (g), height h, the roll centerh_(r) and roll angle φ.

As exhibited above, lateral acceleration plays a role during rollovercrashes. While front forces on the vehicle are discussed, rear forces onthe rear of the vehicle or a combination of front and rear forces on theentire vehicle are equally contemplated. The wheel 24 at the leadingside of the vehicle (e.g., the leading side of the vehicle may includeone wheel at the front of the vehicle or two or more wheels at the frontand rear of the vehicle) may receive an early and larger force than thewheel 22 at the trailing side of the vehicle (e.g., the trailing side ofthe vehicle may include one wheel at the front of the vehicle or two ormore wheels at the front and rear of the vehicle) whether the vehicle isin a trip (e.g., hard or soft) rollover event. For example, a mean valueof the forces acting on both sides of the front of the wheels (e.g.,left and right) or to both sides of the front and rear wheels (e.g.,left and right) may be measured as follows:

$\begin{matrix}{F_{mean} = \frac{F_{zl} + F_{zr}}{2}} & \left( {{EQ}.\mspace{14mu} 5} \right)\end{matrix}$

where, F_(mean) corresponds to a mean value of force at the wheelsbefore a rollover event is detected. F_(zl) may correspond to left wheelreaction forces that act on the left wheel on the front of the vehicleor to two or more left wheels on both the front and rear of the vehicle.Left and right wheel reaction forces act on the wheels on the front andthe rear of the vehicle. F_(zr) may correspond to right wheel reactionforces that act on the right wheel on the front of the vehicle or to twoor more right wheels on both the front and rear of the vehicle. Ingeneral, a wheel reaction force is the load transmitted through thewheel(s) (e.g., the tire, rim and/or suspension components due to thesprung mass). A force index may be calculated by:

$\begin{matrix}{{index}_{F} = \left\{ \begin{matrix}\frac{F_{zl} + F_{zr} - {2F_{mean}}}{F_{mean}} & {{{where}\mspace{14mu} {index}_{f}} \geq 0} \\0 & {{{where}\mspace{14mu} {index}_{f}} < 0}\end{matrix} \right.} & \left( {{EQ}.\mspace{14mu} 6} \right)\end{matrix}$

where, in one example, index_(F) is generally a value that is largerthan zero and less than two. In the event index_(F) is larger than 0.5,then one or both wheels on the left side may be lifted from the groundand the other such wheel(s) on the right side may be in contact with theroad. Likewise, in the event index_(F) is larger than 0.5, then one orboth wheels on the right side may be lifted from the ground and theother such wheel or wheels on the left side may be in contact with theground. If index_(F) is zero, then the vehicle may be considered to beairborn.

FIG. 3 illustrates wheel reaction forces for wheels at the front side ofthe vehicle. Values that correspond to a wheel reaction force at theleading side of the left front wheel (e.g., F_(zl) see EQ. 5) aregenerally shown at 25. Values that correspond to a wheel reaction forceat the trailing side of the left front wheel (e.g., F_(zr), see EQ. 5)are generally shown at 27. Values that correspond to a pre-calculatedaverage of a wheel reaction force (e.g., F_(mean), see EQ. 5) aregenerally shown at 29.

In general, the system and schemes as set forth herein to detect vehiclerollover events may take into account both the index_(F) and lateralacceleration to determine whether the vehicle is experiencing a rolloverevent. The index_(F) and lateral acceleration may provide an earlydetection that the vehicle is experiencing a hard or soft trip rolloverevent. In addition, other characteristics such as the roll rate and theroll angle may serve as an early indication to detect whether thevehicle is experiencing a vehicle rollover.

Referring now to FIG. 4, a rollover detection system 30 in accordance toone embodiment of the present invention is shown. The system 30 includesa restraint controller 32 and a suspension controller 34. A multiplexedcommunication bus 36 is operably coupled between the restraintcontroller 32 and the suspension controller 34 to facilitate datacommunication therebetween. The bus 36 may be implemented as either ahigh or medium speed control area network (CAN) communication data link.The bus 36 may be implemented as any such communication data linkgenerally situated to transmit data between any two controllers in avehicle.

A plurality of wheel reaction force sensing devices 38 a-38 n are inelectrical communication with the suspension controller 34. Each wheelreaction force sensing device 38 a-38 n is generally positioned aboutthe suspension system at each wheel/tire of the vehicle. In general, thewheel reaction force sensing devices 38 a-38 n are generally configuredto sense the force associated with various loads applied at one or morewheels of the vehicle. The suspension controller 34 receives suchinformation to determine the wheel reaction force (e.g., F_(zl) andF_(zr)) for each wheel. In another example, the wheel reaction forcesensing devices 38 a-38 n may measure signals from a pressure sensorand/or acceleration sensor and transmit such data directly to thesuspension controller 34 to determine the wheel reaction force for eachwheel. Other such examples of wheel reaction force sensing devices 38a-38 n may include a pressure sensor positioned in an active airsuspension that may measure the force, a strain gauge, wheel lateralforce sensors, longitudinal wheel force sensors, a vertical tire forcesensor, a tire acceleration sensor, or a tire sidewall torsion sensor.

As noted above in connection with FIG. 2, the suspension controller 34may include F_(mean) (e.g., the mean value of force at the front wheelsand/or the rear wheels of the vehicle before a rollover event) stored inmemory therein. The suspension controller 34 may calculate the forceindex (e.g., see EQ. 6) for the wheels at the front and/or the rear ofthe vehicle in response to such information and transmit suchinformation over the bus 36 to the restraint controller 32.

The restraint controller 32 includes a plurality of accelerometersensors 40 positioned therein. The accelerometer sensors 40 areconfigured to measure car body accelerations about the x-axis(longitudinal acceleration), the y-axis (lateral acceleration), andz-axis (vertical acceleration). For illustrative purposes, a right handcoordinate system may be superimposed on the vehicle. The x-axis of thevehicle may be defined as the axis extending between the fore and aftportions of the vehicle. The positive direction of the x-axis may be thedirection pointing towards the front of the vehicle. The y-axis of thevehicle may be defined as the axis extending from the passenger side ofthe vehicle to the driver side of the vehicle (e.g., the axis extendingthe width of the vehicle). The z-axis of the vehicle may be defined asthe axis extending from the bottom to top of the vehicle. The positivedirections of the y-axis and z-axis are considered to be pointingtowards the driver side and in an upward direction, respectively.

In reference to the lateral acceleration, the accelerometer sensors 40may present hardwired data which correspond to the lateral accelerationof the vehicle body to the restraint controller 32. The restraintcontroller 32 may calculate the lateral acceleration as noted inconnection with EQ. 4. The restraint controller 32 may use the lateralacceleration and the force index as an indicator to determine if thevehicle is in a rollover event. The restraint controller 32 may deploy arestraint system to protect the occupants of the vehicle in response tothe lateral acceleration and the force index exceeding predeterminedthresholds. The restraint system may include curtain and/or side impactairbags that are utilized to protect the occupant.

The restraint controller 32 further includes a roll rate sensor 42positioned therein. The roll rate sensor 42 may measure the roll rate ofthe vehicle. In general, the roll rate of the vehicle is defined as theangular velocity of the vehicle as the vehicle rotates about the x-axisof the vehicle. The restraint controller 32 may calculate the roll angleφ of the vehicle (as discussed in connection with FIG. 2) in response toreceiving the roll rate from the roll rate sensor 42. The restraintcontroller 32 may use the force index and any one or more of the lateralacceleration, the roll angle, and the roll rate of the vehicle todetermine when the vehicle is in a roll over state.

Referring now to FIG. 5, a first rollover detection scheme 50 inaccordance to one embodiment of the present invention is shown. In block52, the restraint controller 32 receives the measured roll rate of thevehicle from the roll rate sensor 42 and filters and conditions suchinformation to determine the roll rate.

In block 54, the restraint controller 32 determines whether the detectedroll rate of the vehicle has exceeded a predetermined roll rate value.Such a value may be stored in memory of the restraint controller 32. Thepredetermined roll rate value may be a calibrated value and vary basedon the type of vehicle that is used. The restraint controller 32compares the roll rate to the predetermined roll rate value. If the rollrate is not greater than the predetermined roll rate value, then thescheme 50 moves to back to block 52. If the roll rate is greater thanthe predetermined roll rate value, then the scheme 50 moves to blocks 56and 58.

In block 56, the restraint controller 32 calculates F_(mean) as notedabove in connection with EQ. 5. The restraint controller 32 receivesF_(zl) and F_(zr) from the suspension controller 34 over the bus 36.

In block 60, the restraint controller 32 calculates the force index asnoted above in connection with EQ. 6.

In block 62, the restraint controller 32 determines the lateralacceleration. The restraint controller 32 calculates the lateralacceleration in response to information transmitted by the accelerometersensors 40 (see EQ. 4).

In block 64, the restraint controller 32 determines whether thecalculated force index and lateral acceleration exceed force indexthresholds and lateral acceleration thresholds, respectively. FIG. 6depicts the force index thresholds and the lateral accelerationthresholds at 90. The force index and lateral acceleration thresholdsare generally calibrated values and may vary (e.g., may be differentthan that shown in FIG. 6) depending on the type of vehicle used. Thethresholds 90 for the force index and later acceleration may be achievedby the following:

$\begin{matrix}{y = {b\left( {1 - \frac{x^{2}}{a^{2}}} \right)}^{c}} & \left( {{EQ}.\mspace{14mu} 7} \right)\end{matrix}$

where variables a and b are derived (or obtained) in response toperforming vehicle rollover tests for a particular vehicle. The variablea and b may vary based on the type of vehicle used. Variables x and ymay correspond to the values of the lateral acceleration and the forceindex, and c is a constant that may vary based on a threshold request.

Values corresponding to the calculated force index and the lateralacceleration that may be indicative of the vehicle being in a rolloverstate is generally shown at 92. Values corresponding to the calculatedforce index and the lateral acceleration that may be indicative of thevehicle not being in a rollover state is generally shown at 94.

In reference to FIG. 5, while blocks 56, 60, 62, and 64 are beingexecuted, blocks 58 and 66 may be executed simultaneously for validationpurposes to confirm that the vehicle is in a rollover state. In general,blocks 58 and 66 may be executed as a secondary measure to ensure thatthe vehicle is in a rollover state.

In block 58, the restraint controller 32 also performs a safingfunction. With such an operation, the restraint controller 32 determinesthe lateral acceleration and the vertical acceleration to determinewhether such values are indicative of the vehicle being in a rolloverstate.

In block 66, the restraint controller 32 determines whether the lateralacceleration and the vertical acceleration exceed predefined safingvalues (e.g., predefined lateral and vertical acceleration safingvalues). The predefined lateral acceleration safing values are valuesthat may or may not be different from the lateral acceleration thresholdas noted in connection with block 64. If the lateral acceleration andthe vertical acceleration do not exceed the predefined safing values,then the scheme 50 moves back to block 58. If the lateral accelerationand the vertical acceleration exceed the predefined safing values, thenthe scheme 50 moves to block 68.

In block 68, the restraint controller 32 determines whether thecalculated force index and the lateral acceleration exceed the forceindex threshold and the lateral acceleration threshold, respectively,and whether the lateral acceleration and the vertical accelerationexceed the predefined safing values. If both conditions have been met,then the scheme 50 moves to block 70. If none or only one of theconditions of blocks 64 and 66 has been met, then the scheme 50 movesback to the start state.

In block 70, the restraint controller 32 deploys the advanced restraintsystem to protect the occupant(s).

Referring now to FIG. 7, a second rollover detection scheme 100 inaccordance to one embodiment of the present invention is shown. Blocks52, 54, 56, 58, 60 and 66 are similar to the blocks 52, 54, 56, 58, 60and 66 of FIG. 5.

In block 102, the restraint controller 32 obtains the roll rate of thevehicle in response to information sent by the roll rate sensor 42.Block 102 is generally similar to the operation performed in block 52.

In block 104, the restraint controller 32 determines whether thecalculated force index and the roll rate exceed the force indexthreshold and the roll rate threshold, respectively. FIG. 8 depicts theforce index threshold and the roll rate threshold at 150. The forceindex threshold and the roll rate threshold are generally calibratedvalues and may vary (e.g., may be different than those shown in FIG. 8)depending on the type of vehicle used. As noted above, the thresholds150 for the force index and roll rate may each be derived by using EQ.7.

Variables x and y as shown in accordance to EQ. 7 may correspond to thevalues of the roll rate and the force index. Variables a and b arederived (or obtained) in response to performing vehicle rollover testsfor a particular vehicle and c is a constant that may vary based on thethreshold requirement (e.g., if c is less than 1, such a condition maycorrespond to an upper lobe 152 as shown in FIG. 8, if c is larger than1, such a condition may correspond to lower lobe 153 as shown in FIG.8).

Values corresponding to the calculated force index and the roll ratethat may be indicative of the vehicle being in a rollover state aregenerally shown at 152 in FIG. 8. Values corresponding to the calculatedforce index and the roll rate that may be indicative of the vehicle notbeing in a rollover state are generally shown at 154 in FIG. 8.

In block 106, the restraint controller 32 determines whether thecalculated force index and the roll rate exceed the force indexthreshold and the roll rate threshold and whether the lateralacceleration and the vertical acceleration exceed predefined safingvalues. If both conditions have been met, then the scheme 100 moves toblock 108. If none or only one of the conditions has been met, then thescheme 100 moves back to the start state.

In block 108, the restraint controller 32 deploys the advanced restraintsystem to protect the occupant(s).

In another embodiment, references to the roll rate as noted inconnection with blocks 102, 104, and 106 may be replaced with the rollangle. For example, the block 102 may calculate the roll angle of thevehicle as opposed to the roll rate of the vehicle. The roll angle isgenerally defined as angular rotation of the vehicle body. The roll rateis generally the angular velocity of the vehicle body. In contrast tohaving a roll rate threshold (see block 104), the roll angle thresholdmay be established along with the force index threshold. In such anexample, the restraint controller 32 may determine whether thecalculated force index and the calculated roll angle exceed the forceindex threshold and the roll angle threshold, respectively, as shown at200 in FIG. 9. FIG. 9 depicts the force index threshold and the rollangle threshold at 200. The force index threshold and the roll anglethreshold are generally calibrated values and may vary (e.g. may bedifferent than that shown in FIG. 9) depending on the type of vehicleused. The thresholds 200 for the force index and the roll angle may eachbe derived by EQ. 7 as stated above.

Values corresponding to the calculated force index and the roll anglethat may be indicative of the vehicle being in a rollover state isgenerally shown at 202. Values corresponding to the calculated forceindex and the calculated roll angle that may be indicative of thevehicle not being in a rollover state is generally shown at 204.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

1. An apparatus comprising: a controller operably coupled to at leastone accelerometer sensor that transmits a first acceleration signal, thecontroller configured to: determine a lateral acceleration based on thefirst acceleration signal; determine a force index based on an amount offorce exerted on a wheel; compare the force index to a threshold forceindex and the lateral acceleration to a threshold lateral acceleration;and deploy a restraint based on the comparison of the force index to thethreshold force index and the lateral acceleration to the thresholdlateral acceleration.
 2. The apparatus of claim 1 wherein the controlleris further configured to perform a safing function based on the firstacceleration signal prior to deploying the restraint.
 3. The apparatusof claim 2 wherein the at least one accelerometer sensor is furtherconfigured to transmit a second accelerometer signal and the controlleris further configured to determine a vertical acceleration in responseto the second accelerometer signal and to perform the safing functionwith the lateral acceleration and the vertical acceleration.
 4. Theapparatus of claim 1 wherein the force index corresponds to a firstforce measurement at one or more left wheels of the vehicle, a secondforce measurement at one or more right wheels of the vehicle, and anaverage force between the one or more left wheels and the one or moreright wheels.
 5. The apparatus of claim 1 wherein the controller furtherincludes at least one roll rate sensor for transmitting at least oneroll rate signal indicative of an angular velocity of the vehicle. 6.The apparatus of claim 5 wherein the controller is further configuredto: determine a roll angle in response to the roll rate signal; comparethe roll angle to a threshold roll angle; and deploy the restraint inresponse to a comparison of the first force index to the threshold forceindex and the roll angle to the threshold roll angle.
 7. The apparatusof claim 5 wherein the controller is further configured to: determine aroll rate in response to the roll rate signal; compare the roll rate toa threshold roll rate, and deploy the restraint in response to acomparison of the first force index to the threshold force index and theroll rate to the threshold roll rate.
 8. A vehicle rollover apparatuscomprising: a controller operably coupled to at least one roll ratesensor that transmits a roll rate signal, the controller configured to:determine a roll rate of a vehicle based on the roll rate signal;determine a force index based on an amount of force exerted on a wheel;compare the roll rate to a threshold roll rate and the force index to athreshold force index; and deploy a restraint based on the comparison ofthe roll rate to the threshold roll rate and the force index to thethreshold force index.
 9. The apparatus of claim 8 further comprising atleast one accelerometer sensor for transmitting a first accelerationsignal indicative of a vehicle body acceleration about at least one axisof the vehicle.
 10. The apparatus of claim 9 wherein the controller isfurther configured to: determine a lateral acceleration in response tothe first acceleration signal; compare the lateral acceleration to athreshold lateral acceleration; deploy the restraint in response to thecomparison of the first force index to the threshold force index and thelateral acceleration to the threshold lateral acceleration.
 11. Theapparatus of claim 9 wherein the controller is further configured toperform a safing function based on the lateral acceleration prior todeploying the restraint.
 12. The apparatus of claim 11 wherein the atleast one accelerometer sensor is further configured to transmit asecond accelerometer signal and the controller is further configured toperform the safing function with the first acceleration signal and thesecond acceleration signal.
 13. The apparatus of claim 8 wherein theforce index corresponds to a first force measurement at one or more leftwheels of the vehicle, a second force measurement at one or more rightwheels of the vehicle, and an average force between the one or more leftwheels and the one or more right wheels.
 14. A method for detecting arollover of a vehicle, the method comprising: receiving a roll ratesignal indicative of a roll rate of the vehicle, determining a rollangle based on the roll rate signal; determining a force index based onan amount of force exerted on a wheel; comparing the force index to athreshold force index and the roll angle to a threshold roll angle; anddeploying the restraint responsive to the comparison.
 15. The method ofclaim 14 wherein the roll rate is indicative of an angular velocity ofthe vehicle.
 16. The method of claim 14 wherein the roll angle isindicative of an angular rotation of the vehicle.
 17. The method ofclaim 14 further comprising providing a first acceleration signalindicative of a vehicle body acceleration about at least one axis of thevehicle.
 18. The method of claim 17 further comprising: determining alateral acceleration in response to the first acceleration signal;comparing the lateral acceleration to a threshold lateral acceleration;and deploying the restraint in response to the comparison of the firstforce index to the threshold force index and the lateral acceleration tothe threshold acceleration.
 19. The method of claim 17 furthercomprising performing a safing function based on the first accelerationsignal prior to deploying the restraint.
 20. The method of claim 19further comprising: providing a second accelerometer signal; andperforming the safing function with the first acceleration signal andthe second acceleration signal.